The subject invention relates to through air drying for tissue paper papermaking, and more particularly to through air drying usable with micropore drying media.
Micropore drying media are known in the art. Micropore drying media include a ply, or a plurality of plies superimposed in face-to-face relationship. The plies provide restrictions in the flow path for air flow therethrough. The restrictions in the flow path may comprise pores smaller than many of the interstitial areas in tissue paper, as well as other generally planar materials dried, or otherwise made, thereon. The following discussion is directed to tissue paper, it being understood that the invention is not so limited.
By providing pores smaller than the interstices of the tissue paper, differences in flow resistance through the tissue paper, etc., are negated due to the greater flow resistance being provided by the micropore drying medium. Such differences in flow resistance may occur due to differences in intensive properties, such as caliper, basis weight and density. Typically, such differences occur on a very small scale due to localized differences in the various regions of the tissue paper.
The prior art discloses micropore media suitable for drying tissue paper thereon. Improvements to the micropore media include micropore drying apparatus having multiple zones, high fatigue strength/low pressure drop micropore drying media and micropore media having preferentially reduced wet pressure drop. Such micropore media, suitable for adaptation to the present invention, are illustrated in commonly assigned U.S. Pat. No. 5,274,930, issued Jan. 4, 1994 to Ensign et al.; U.S. Pat. No. 5,437,107, issued Aug. 1, 1995 to Ensign et al.; U.S. Pat. No. 5,539,996, issued Jul. 30, 1996 to Ensign et al.; U.S. Pat. No. 5,581,906, issued Dec. 10, 1996 to Ensign et al.; U.S. Pat. No. 5,584,126, issued Dec. 17, 1996 to Ensign et al.; U.S. Pat. No. 5,584,128, issued Dec. 17, 1996 to Ensign et al.; U.S. Pat. No. 5,625,961, issued May 6, 1997 to Ensign et al.; U.S. Pat. No. 5,912,072, issued Jun. 15, 1999 to Trokhan et al.; U.S. Pat. No. 5,942,322, issued Aug. 24, 1999 to Ensign et al.; U.S. Pat. No. 6,021,583, issued Feb. 8, 2000 to Stelljes, Jr. et al.; and U.S. Pat. No. 6,105,276, issued Aug. 22, 2000 to Ensign et al., which patents are incorporated herein by reference.
There remain other ways to optimize energy consumption when using micropore drying techniques. For example, as water is removed from the tissue paper, etc., to be dried by air flow therethrough, subsequent flow restrictions in the micropore media need not be as great. Thus, in the machine direction, flow restrictions in the micropore media may be reduced while maintaining a pore size smaller than many, preferably most, and most preferably all, of the interstices in the tissue paper.
Thus, flow restrictions through the micropore media may be reduced as the tissue paper to be dried travels across the micropore drying medium in the machine direction. This arrangement provides the benefit of decoupling mechanical dewatering of the tissue paper from through air drying of the tissue paper. During mechanical dewatering, a small pore size is better to promote dewatering by capillary action. During through air drying, pore sizes which are relatively larger, but still provide a limiting orifice for air flow through the tissue paper, have less flow resistance and thereby save energy.
Reduced flow resistance through the micropore media may be provided by having pore sizes which successively increase in the machine direction. Alternatively, micropore media having a higher density of pores, i.e., more pores per square centimeter, in the machine direction may be utilized. Finally, hybrid media having both of the above features may be utilized. Furthermore, the reduced flow resistance apparatus of the present invention may be used with through air drying tissue paper papermaking processes which are not limited to micropore drying media. The variable flow resistance apparatus and process according to the present invention may be applied to other through air drying tissue paper papermaking techniques as well. For example, the disclosed apparatus and process may be used with the predryers of a through air drying tissue paper papermaking machine.
The invention comprises a micropore drying apparatus having a machine direction and a Z-direction orthogonal thereto. The micropore drying apparatus is permeable to air flow therethrough. The micropore drying apparatus has a wet flow resistance to air flow therethrough, which wet flow resistance to air flow decreases in the machine direction of the micropore drying apparatus.
The micropore drying apparatus has a grid of pores which provide the air flow therethrough. The wet flow resistance may decrease in either a step wise fashion or in a gradient. The decrease may occur within sections of, or entirely throughout, the drying apparatus. The decreased flow resistance may be achieved by increasing the size and/or number of pores. In yet another embodiment, the decreasing pore resistance may be provided by coating the micropore drying apparatus to reduce the surface energy, or changing the flow path through the pores to be less tortuous and provide a lesser flow resistance in the Z-direction. In yet another embodiment, the hydraulic radius of the pores may be reduced.